There was also some doc on the usage of the "getRealPath" which could be used to obtain the path for the properties file name:

Web applications
The Liberty profile server does not automatically expand web archive (WAR) files
that are deployed to the server. The Java EE specification states that the
getRealPath() method returns a null value if the content is being made available
from a WAR file.
If your application relies on a result being returned by getRealPath(), you must
deploy the application as an expanded web application, not as a WAR file. For
example, you can manually extract the WAR file and copy the expanded
application to the dropins directory.

If neither of the above provides a solution a couple of thoughts:

a.) If you have not already, you might want to post something on a WAS or WAS Liberty forum.

b.) As you probably already know and as I remember it, the properties files are searched for on the CLASSPATH. So, you could just allow them to be placed in the root folder of the generated Java files (wrappers) and deploy them as part of the WAR with no special entries. Just a thought if it works for you.